1. Field of the Invention
The present invention relates to a Radio Frequency IDentification (RFID) system, and more particularly to an apparatus and a method for improving receive (Rx) sensitivity in a portable RFID reader/writer.
2. Description of the Related Art
Development of fixed-type RFID technology, which had been used in the circulation of products, etc., has brought about appearance of mobile RFID for individual use of information, such as in goods, movies, music, and others. The term “mobile RFID” signifies that a built-in RFID function is equipped in a mobile terminal, such as a mobile phone, and is referred to as “portable RFID.”
Conventionally, an RFID system consists of a reader and a tag. The tag is not equipped with its own power supply. Instead, the tag generates electric power from a Continuous Wave (CW), i.e. a carrier signal, received from the reader, and then uses the generated power. Because the tag has no power supply of its own, the reader transmits to the tag an instruction and a data signal together with a CW, and then transmits another CW corresponding to a source of power supply for the tag while the tag is responding to the transmitted instruction, the data signal, and the CW. While waiting for a response from the tag following the transmission of the instruction, the reader transmits a CW, and then transmits another CW of significantly high power so that the tag may have enough power. During this process the reader is greatly affected, the degree of which depending on the amount of a leakage current of a transmit (Tx) signal during a receive (Rx) operation in which the reader receives a signal from the tag.
One difficult point in implementing an RFID system is that the Tx signal uses the same frequency as the Rx signal. A Tx signal having a strong electric field which has leaked to an Rx path cannot be removed by using a filter, such as a Radio Frequency (RF) Surface Acoustic Wave (SAW) filter. Therefore, only if the amount of the leakage current of the Tx signal which flows into the Rx path is reduced, Rx sensitivity can be improved.
As illustrated in FIG. 1, paths along which a Tx signal flows into an Rx path are classified into two kinds of paths in an RFID reader. The first path corresponds to a path along which the Tx signal leaks to the Rx path due to the isolation characteristic of a directional coupler 104, arranged in the next stage of a Power Amplifier Module (PAM) 102. The second path corresponds to a path along which the Tx signal reflected from an antenna stage of an antenna 100 flows into the Rx path, due to the coupling characteristics of the directional coupler 104.
The isolation characteristic of the directional coupler 104 usually corresponds to an extent of −35 dBm. Accordingly, assuming the power of the Tx signal output from the PAM 102 equals +30 dBm, a power of −5 dBm (i.e., −35 dBm+30 dBm=−5 dBm) passes through the directional coupler 104, and then flows into the Rx path. On the other hand, the reflection coefficient of the antenna 100 usually equals −10 dBm. Hence, after the Tx signal having a power of +30 dBm is reflected from the antenna 100 at a level of +20 dBm, a power of +5 dBm (i.e., 30 dBm−10 dBm−15 dBm=+5 dBm) flows into the Rx path according to the coupling characteristic of −5 dBm of the directional coupler 104. Therefore, reducing the reflecting coefficient of the antenna 100 is essential to improve the Rx sensitivity of the RFID system.
Meanwhile, since the tag does not have its own power supply but uses a received Tx signal from the reader as a source of electricity in the RFID system, only if the reader is equipped with an antenna having a good gain, the reader may transmit enough power to the tag and fully receive a weak signal delivered from the tag. In conditions where most noises occurring in the RFID system (i.e., Tx signal components, isolated by the directional coupler 104, and Tx signal components, which return upon reflecting from the antenna 100 and then couple to one another) have been determined based on unique characteristics of both the directional coupler 104 and the antenna 100, Rx sensitivity can be improved by using an antenna having a good gain.
Since existing fixed-type RFID reader/writers and reader-only terminals have little restrictions on the size of an antenna and can use antennas having good gain, Rx sensitivity thereof are good, and the probability and the distance of reading a tag are also good. On the other hand, antennas are sensitive to external influences. For example, if a person's hand touches an antenna, or is located nearby, the characteristics of the antenna would change, and especially, the variation of its reflection coefficient would be great. Since in fixed-type antennas or reader-only terminals, the distances between external influences such as persons' hands and the antennas are long the antennas in those systems are hardly affected by such external factors.